In MSK2, the phosphorylation of the two probable MAPK sites, Thr-568 and Ser-343, from the upstream kinase (ERK or p38) has been shown to be required for activity. consequently for the phosphorylation of MSK1 substrates sites for ERK1/2, the upstream activator of RSK in cells. ERK1/2 is definitely recruited to RSK through a C-terminal docking sequence, and mutation of this sequence prevents the activation of RSK by ERK1/2 [20C22,24,25]. Mutation of Ser-381 exposed that phosphorylation of this residue was also involved in RSK activation. Ser-381 was found to be phosphorylated from the C-terminal website of RSK and this phosphorylation was critical for the activation of the N-terminal kinase website. Ser-222 phosphorylation was also found to be required for the activity of the N-terminal kinase website; however, the mechanism of Ser-222 phosphorylation in RSK was not determined by these initial studies. It was proposed, based on these results, that activation of RSK required the phosphorylation of Ser-364 and Thr-574 by ERK1/2, which in turn triggered the C-terminal kinase website that phosphorylated Ser-381. The phosphorylation triggered the N-terminal kinase website, which could then phosphorylate the substrates [23,26]. This model has now been refined due to improvements in the understanding of the activation of AGC kinase family members. Most AGC kinases require the phosphorylation of two sites for full activity, a serine or threonine residue in the T-loop sequence (Ser-222 in RSK) and a serine residue (Ser-381 in RSK) inside a sequence termed the hydrophobic motif, which lies C-terminal to the AGC kinase website [27,28]. Phosphorylation of the T-loop residue in AGC kinases is required for activity, and mutation of this residue to alanine in all the AGC kinase family members tested to Tiaprofenic acid day prevents activation of the kinase. For any subset of AGC kinases, including RSK, the T-loop residue is definitely phosphorylated by PDK1 (3-phosphoinositide-dependent protein kinase 1) [29C31]. PDK1 appears to be active constitutively in cells, and T-loop phosphorylation of AGC kinases by PDK1 is definitely controlled in two major ways depending on the AGC kinase Tiaprofenic acid involved . PDK1 was first identified as the upstream activator of PKB. The phosphorylation of PKB by PDK1 is dependent on phosphoinositide 3-kinase activity. The activation of phosphoinositide 3-kinase in the membrane results in the production of PIP3 (phosphatidylinositol 3,4,5-trisphosphate). Both PKB and PDK1 are recruited to the membranes from the binding of their PH (pleckstrin homology) domains to PIP3. The binding of PIP3 to the PH domains is necessary to allow PDK1 to phosphorylate the T loop of PKB. Consistent with this, mutation of the PH website in PDK1 helps prevent the activation of PKB in cells. In contrast, for additional AGC kinases, including RSK and p70S6K (p70 ribosomal S6 kinase), the phosphorylation of the T-loop residue by PDK1 is definitely controlled from the hydrophobic motif in the C-terminus of the AGC kinase website [32,33]. Phosphorylation of the hydrophobic motif introduces a negative charge to this motif, which RFC4 enables the recruitment of PDK1 through the phosphate binding in the PIF (PDK1 interacting fragment) pocket of PDK1. Tiaprofenic acid Once bound, PDK1 is definitely then capable of phosphorylating the T loop of the AGC kinase. The requirement of PDK1 for the phosphorylation of Ser-222 of RSK, and its subsequent activation, has been shown in cells by the use of PDK1C/C embryonic stem cells and a knock-in mutation that helps prevent PDK1 binding to the phosphorylated hydrophobic motif of AGC kinases [30,33]. It has been demonstrated that MSKs are triggered in cells by either the ERK1/2 or p38 pathways depending on the stimuli used, and that inhibition of these pathways blocks MSK activation in cells. In MSK2, the phosphorylation of the two probable MAPK sites, Thr-568 and Ser-343, from the upstream kinase (ERK or p38) offers been shown to be required for activity. Furthermore, on the basis of mutational analysis , it has been found that phosphorylation of at least two further sites, Ser-196 and Ser-360 (the T loop and hydrophobic motif), is also required for MSK2 activity. Less is definitely, however, known about the activation of MSK1. Mutation of either the C- or N-terminal kinase domains in MSK1 offers been shown to be sufficient to block the phosphorylation of MSK substrates . However, the mechanism of Tiaprofenic acid MSK1 activation has not been analyzed further, but was initially believed to.